Automatic vehicle monitoring system

ABSTRACT

A cooperative fleet vehicle location monitoring system utilizes low-energy-level coded impulse transmissions characterizing possible vehicle locations along a route to permit an impulse receiver aboard the cooperating vehicle to cause generation of coded transmissions receivable at a headquarters control location repeatedly identifying the vehicle and its location.

Unite States Patent 11 1 on 3,757,290 Ross et a1. 1 1 Sept. 4, 1973 [541AUTOMATIC VEHICLE MONITORING 2,904,674 9/1959 Crawford 343/100 (:5SYSTEM 3,568,161 3/1971 Knickel 340/24 3,474,460 10/1969 Hucbscher....343/6.8 R 1 lnventorsr Gerald E Ross, g 3,377,616 4/1968 Auer, Jr 340 23John J. Morrone, Rego Park; 3,419,865 12/1968 Chisholm... 340/24 WilliamW. Bell, III, Sands Point, both of NY OTHER PUBLICATIONS Palatnick andInhelder, AVI Systems-Methods of Ap- [73] Asslgnee' i g 5$ CorporationNew proach, IEEE Transactions on Vehicular Technology,

or V01. 19, No. 1, February 1970, Part of Article. [22] Filed: Mar. 12,1971 Appl. No.: 123,516

US. Cl 340/23, 325/117, 343/100 CS Int. Cl G08g l/12 Field of Search340/22, 23, 24, 31 R, 340/32, 33, 38 R, 38 S; 343/6.5 R, 6.8 R, 6.5 SS,100 CS, 112 R, 112 PT, 112 TC; 179/41 A;325/16, 117

References Cited UNITED STATES PATENTS l-Ieibel 250/199 Borman et al.340/23 Primary ExaminerKathleen H. Claffy Assistant ExaminerRandall P.Myers Att0rneyS. C. Yeaton [57] ABSTRACT A cooperative fleet vehiclelocation monitoring system utilizes low-energy-level coded impulsetransmissions characterizing possible vehicle locations along a route topermit an impulse receiver aboard the cooperating vehicle to causegeneration of coded transmissions receivable at a headquarters controllocation repeatedly identifying the vehicle and its location.

10 Claims, 14 Drawing Figures PAniIIriIIs'tr'mn SHEEI on;

HIGH 1 PASS f FILTER voIcE H AUDIO TRANSCEIVER SYSTEM ONE SHOT ANDMULTIVIBRATOR GATE COUNTER ENCODER INTERROGATOR 78 GERALD F. ROSS JOHNJ. W/LL/AM B) MORRO/VE W. BELL 3RD 1 AUTOMATIC VEHICLE MONITORING SYSTEMBACKGROUND OF THE INVENTION 1. Field of the Invention The inventionpertains to apparatus permitting rapid automatic monitoring of thelocation of cooperating vehicles of a fleet and more particularlyrelates to an impulse communication system for identifying the locationof each cooperatively equipped vehicle as it passes selectedelectronically instrumented locations on its route.

2. Description of the Prior Art Operators of fleets of vehicles in urbanenvironments, including emergency vehicles, have need of a generalpurpose system for rapid monitoring at a control headquarters locationof the locations of fleet vehicles. Typical operators facing the needare operators of fleets of vehicles such as police, fire, bus, taxi,delivery truck, ambulance, armored carrier, mass transport, utilityrepair, and security guard patrol vehicles. Rapid location monitoringhas not been available for effective .fleet management, roll call,scheduling and headway control, optimum dispatching, priority routing,and crime deterrence. Effective means for the monitoring of locations oflarge vehicle fleets, often totalling as many as a thousand vehicles andoften scattered over large urban areas, has not been possible. Even thewidely spread call box system and two-way radio transmission systemsemployed in emergency vehicles and sometimes in mass transportationcarriers are expensive and time consuming to use. Reporting at frequentintervals distracts the vehicle operator from attention to properoperation of his vehicle and is therefore unsuitable in emergencysituations. Yet it is always in major emergency situations thateffective monitoring is most needed, for instance, to bring police orother vehicles into proper convergence to surround a region of majordisturbance, or to route fire equipment to a major fire safely alongseparated routes so that collisions between fire fighting vehicles arenot risked. Fully effective monitoring of the locations of elements ofcommercial fleets is clearly also desirable as a tool permitting maximumeconomic use and maximum highjack resistant operation of such commercialvehicles.

SUMMARY OF THE INVENTION The invention is an impulse radio communicationsystem using low-energy-level coded impulse transmitters and impulsereceivers for signalling the presence of fleet vehicles at selectedlocations as they progress along a route. Coded fleet vehicle identityand location data is transmitted from the vehicle to a centralheadquarters location from which instructions may be issued toindividual vehicle drivers over conventional broadcast equipment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of atypical urban intersection equipped to operate according to the presentinvention.

FIG. 2 is a map of a network of streets useful in explaining theoperation of the invention.

FIG. 3 is a perspective view, partly in cross section, showing theexternal appearance of an impulse transmitter-antenna configuration usedin the invention.

FIG. 4 is an equivalent circuit of the apparatus of FIG. 3.

FIGS. 5a, 5b, 6a, 6b, 7a, 7b, 8a, and 8b are graphs useful in explainingthe operation of the transmitterantenna configuration of FIGS. 3 and 4.

FIG. 9 is a block diagram of a preferred impulse receiver for use in theinvention.

FIG. 10 is an alternative form of an impulse antenna for use in theimpulse receiver of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates arepresentative situation in which a cooperating fleet vehicle 6 ispresent in an urban street intersection area illuminated continuously bya train of electromagnetic impulse transmissions provided by an impulsetransmitter-antenna configuration I mounted for example, in the novelsystem on a street lamp standard 2 supported by a street lamp pole 3.Other transmitter-antenna configurations like configuration 1 may belocated at other street intersections or at any other selected arealocation which may be traveled over by the cooperating fleet vehicle 6.If desired, directive transmitter-antenna configurations like device 1may be suspended from buildings and may otherwise be arranged to behidden or unrecognized by those having no basis for knowing of theirpresence. Each transmitter-antenna configuration is characterized byemitting impulses having an impulse repetition frequency peculiar to itsparticular location.

A cooperating fleet vehicle, such as the emergency vehicle 6, is equppedwith a radome-protected antenna 8 specially designed for receiving theimpulse transmissions of each transmitter-antenna configuration l as thearea illuminated by the latter is traversed by the vehicle. Reception,therefore, of an impulse wave train of a particular repetition frequencyidentifies the location being traversed by the vehicle at the moment ofreception. Reception of the impulse wave by novel radio receiverequipment coupled to antenna 8 may cause broadcast of identificationinformation by a conventional communication transceiver antenna 7 to acentral headquarters, thus informing such a central command postinstantaneously of the presence of emergency vehicle 6 at, for example,the intersection of 9th Street and I Street.

As seen in FIG. 2, the pole 3 on which the transmitter-antennaconfiguration l of FIG. 1 is located at 9th Street and I Street isplaced on one corner of a common type of right angle street intersectionin such a way that impulse radiation from the transmitter-antenna 1generally covers an area indicated for convenience by circular boundary3a, a boundary which generally may be other than truly circular. It isto be understood that the center of the illuminated area will notnecessarily define the location of the transmitter-antenna 1. If IStreet is to be furnished with a generally regular array of monitoredintersections, other directive antennatransmitter apparatus 1 accordingto FIGS. 3 and 4 may be placed on street lamp poles l0 and 11, forexample, for illuminating the respective areas 10a and 11a with impulseradiation. It is thus seen that the successive intersections of I Streetwith 9th, 10th, and 11th Streets are served and that a cooperatingvehicle moving along I Street will enter areas of illumination byimpulse energy of successively different impulse repetition frequencies.The area of effective illumination, for example, at the 9th Street and IStreet has a maximum dimension of substantially 200 feet.

Such an area of illumination is also sufficient, for example, to coverthe various proximate intersections of I Street, th Street, andBoulevard A from a centrally placed transmitter-antenna system 1 onstreet light pole 10. If more of Boulevard A is to be serviced by thesystem, non-overlapping illumination areas 12a and 13a may be similarlyproduced at the intersections of Boulevard A and J Street and ofBoulevard A and 11th Street. A cooperating vehicle passing alongBoulevard A will then meet successive impulse energy illuminated areas12a, 10a, and 13a, each having a distinctive impulse repetitionfrequency.

An antenna-transmitter system for use as configuration 1 in the novelsystem of FIG. 1 is of a special type to be discussed in connection withFlGS. 3 and The configuration l employs an electrically smooth, constantimpedance, transmission line system for propagating TEM modeelectromagnetic waves. The transmission line system which is animprovement over that disclosed in the G.F. Ross et al. Pat. applicationSer. No. 46,079 for a Balanced Radiator System," filed June 15, 1970,issued Apr. 25, 1972 as US. Pat. No. 3,659,203, and assigned to theSperry Rand Corporation, is employed for the cooperative cyclic storageof energy on the transmission line and for its cyclic release bypropagation along the transmission line formed as a flared or tapereddirective antenna. Thus, cooperative use is made of the transmissionline system for signal generation by cyclically charging thetransmission line at a rate determined by the distributed capacity C andresistors 51 and 51a, as will be seen, and for signal radiation intospace by discharge of the line in a time much shorter than required forcharging. Discharge of the transmission line causes a voltage wave totravel toward the open end or radiating aperture of the antennastructure. The process operates to produce, by differentiation, a sharpimpulse that is radiated into space. The antenna system has a wideinstantaneous bandwidth, so that it may radiate very sharp impulse-likesignals with low distortion. Further, the antenna has an energy focusingcharacteristic such that energy radiated in predetermined direction ismaximized.

The antenna-transmitter configuration l of FIGS. 3 and 4 comprises astructure having mirror image symmetry about a median plane at rightangles to the direction of the vector of the electric field propagatingwithin the antenna. The same is true of the cooperating transmissionline 30 which comprises parallel plate or slab transmission lineconductors 31 and Ella of similar shape. Conductors 311 and 31a arespaced planar conductors constructed of a material capable of conductinghigh frequency currents with substantially no ohmic loss. Further,conductors 3B and Slla are so constructed and arranged as to support TEMmode propagation of high frequency energy, with the major portion of theelectric field lying between conductors 3i and 31a and with the electricfield substantailly perpendicular to the major interior surfacesthereof.

The TEM transmittenantenna ll further consists of a pair of flared,flat, electrically conducting planar members 32 and 32a. Members 32 and32a are, for example, generally triangular in shape, member 32 beingbounded by flared edges 33 and 33a and a frontal aperture edge 3%.Similarly, member 32a is bounded by flaring edges 35 and 35a and afrontal aperture edge 34a. Edges 34 and 34a may be straight or arcuate.Each of triangular members 32 and 32a is slightly truncated at its apex,the truncation being so constructed and arranged that conductor 31 issmoothly joined without overlap at junction 36 to antenna member 32.Likewise, conductor 31a is smoothly joined without overlap at junction36a to antenna member 32a. It is to be understood that the respectivejunctions 36 and 36a are formed using conventionally availabletechniques for minimizing any impedance discontinuity corresponding tothe junctions 36 and 3601.

it is also to be understood that the flared members 32 and 32a ofantenna 1 are constructed of material highly conductive to highfrequency currents. It is further apparent that the interior volume oftransmitter-antenna 1 may be filled with an air-foamed dielectricmaterial exhibiting low loss in the presence of high frequency fields.The interior of transmission line 30 may be similarly filled withdielectric material, such material acting to support conductor 31 infixed relation to conductor 31a and, likewise, the flared antenna member32 relative to flared member 3211. Alternatively, the conductiveelements of transmission line 30 and transmitterantenna 1 may be fixedin spaced relation by dielectric spacers which cooperate in formingenclosing walls for the configuration, protecting the interiorconducting surfaces of antenna-transmitter configuration 1 from theefiects of precipitation and corrosion. For example, thin vertical walls38 and 38a of low loss dielectric sheet material may be used inconjunction with transmission line conductors 31 and 31a. Side walls forseparating the horn elements 32 and 32a may take the form of triangularlow loss dielectric wall elements 39 and 39a; such side walls, incooperation with a thin front or radome wall 40 of low loss dielectricmaterial, lend mechanical strength to the transmitter-antennaconfiguration 1 and aid in protecting the interior thereof. It will beunderstood that the elements 32 and 32a forming the antenna aperture maybe exponentially tapered, as indicated in FIG. 4, as well as lineallytapered.

A form such as that of the transmission line 30 and thetransmitter-antenna l as illustrated in FIG. 3 is preferred, in part,because TEM mode propagation therein is readily established. The TEMpropagation mode is preferred, since it is the substantiallynon-dispersive propagation mode and its use therefore minimizesdistortion of the propagating signal to be transmitted. The simple,balanced transmission line structure permits construction of theconfiguration l with minimum impedance discontinuities. Furthermore, itis a property of the symmetric type of transmission line ofantennatransmitter ll that its characteristic impedance is a function of11/12, where b is the width dimension of the major surfaces ofconductors 32, 32a and h is the distance between the inner faces of theconductors 32 and 3211. For example, the ratio b/h is kept constant inthe instance of transmission line 30 because both b and h are constant.

According to the invention, the transmitter-antenna l is made compatiblewith transmission line 30 by using the same value of the ratio b/h forboth elements. In other words, if the ratio 12/): is kept constant alongthe direction of propagation in transmitter-antenna l, thecharacteristic impedance of transmitter-antenna 1 will be constant alongits length and may readily be made equal to that of line 30. Bymaintaining a continuously constant characteristic impedance along thestructure including line 30 and transmitter-antenna 1, frequencysensitive reflections are prevented therein. It has been elected, forthe sake of simplicity of explanation, to show in FIG. 3 triangularflaring planar configurations for elements 32 and 32a. It should beevident, however, that other configurations may readily be realizedwhich maintain a constant characteristic impedance according to theabove rule, and that such configurations may also be used within thescope of the present invention.

The system for exciting the transmitter-antenna I of FIG. 3 hascompatible properties, such as being balanced in nature and as avoidingthe complicating deficiencies of an interface balun or other transitionelement. The system of FIG. 4 achieves such objectives and, in addition,makes beneficial use of the balanced dual element configuration oftransmitter-antenna l as part of the charging line for the excitationgenerator. It will be understood that certain liberties have been takenin the drawing of FIG. 4 better to explain the structure and operationof the device disclosed therein. For example, it is seen that FIG. 4 isintended schemati cally to indicate conductor elements 32 and 32a ofFIG. 3 as respective single wire transmisison lines 42 and 420 havingthe same effective electrical characteristics as elements 32 and 32a ofFIG. 3 and the same radiating characteristic. As a further example,junctions 36 and 36a in FIG. 3 are represented by junctions 46 and 46ain FIG. 4. The symbols 31 and 31a in FIG. 3 are represented in FIG. 4 bysymbols 41 and 41a and identify the opposed conductors of transmissionline 30. Dimensions in FIG. 4 are exaggerated, such as the spacing hbetween conductors 41 and 41a of line 30, as a matter of convenience.

At the left end of line 30, conductors 41 and 41a are joined by a seriescircuit comprising battery 50 coupled between charging resistors 51 and51a each having a resistance value R/2 ohms. At the end of line 30adjacent junctions 46 and 46a, the conductors 41 and 41a are joined by aseries circuit comprising an electrically actuable switch 52, which maytake the form of an avalanche transistor or other transistor switch;thus, transistor 52 is coupled across battery 50 through resistors 51,51a, 56, and 56a. Also coupled across battery 50 is an astablemultivibrator 54 which is connected through capacitor 53 to the base oftransistor 52 for the purpose of controlling the state of conduction oftransistor 52. Resistors 56 and 56a each have a resistance value of r/2ohms, where r is equal to the characteristic impedance of line 30 (andof the transmission line comprising elements 42 and 42a). Transistor 52is also provided with a base-to-ground resistor 53a.

Astable multivibrator or pulse generator 54 produces a regular bipolarwave train such as wave 55, of a predetermined pulse repetitionfrequency for actuation of transistor switch 52. In operation, it willbe observed that transistor switch 52 is first held non-conducting bypulse generator 54 for a time sufficient for the entire structureincluding the conductors of line 30 and conductors 42 and 42a to becomecharged to a potential difference V equal to that supplied by battery 50as if charging an effective capacitor C On the next cycle of wave 55,transistor switch 52 is rendered conducting, forming a conductingcircuit path through resistors 56 and 56a. The effect is that of puttinga second or efiective source B in series with the first source A orbattery 50, but reversed in polarity relative to the polarity of thefirst source A.

FIGS. 5a, 6a, 7a, and 8a show the positive voltage V, contributed by thesource A or battery 50, as a positive constant voltage at successiveintervals in the operating cycle. The same set of figures shows theprogress of the negative wave due to the second or effective source B atthe same successive intervals. For example, FIG. 5a shows the situationat the instant switch 52 is rendered conductive; note that the wave dueto the effective second source B has not started to flow.

In FIG. 6a, however, the negative wave of voltage -V/2 from theeffective second source B has begun to flow toward the aperture oftransmitter-antenna 1. Upon reaching the ends 44, 44a of conductors 42and 42a of FIG. 4, and upon being reflected, the situation is depictedin FIG. 7a. It is seen that when the V/2 wave reaches the respectiveends 44, 44a of antenna conductors 42 and 42a, it is reflected andbegins to flow back toward junctions 46, 46a. The total contribution ofthe second of effective source B, beginning at the instant of reversal,is now V volts. It will be seen that the total potential due to the realand the effective sources A and B between conductors 42 and 42a at theaperture 44, 44a of the antenna at the instant of reversal suddenlydrops from +V volts to zero; this instant of time is one of primaryinterest in the operation of the transmitter-antenna 1. The wave due tothe effective source B continues to travel back toward junctions 46, 46auntil the antenna conductors 42, 420, which have served as part of thecharging line for the system are substantially completely discharged, ifthe value of r is the characteristic impedance of the line comprisingconductors 42, 42a. The charging cycle is then reestablished when pulsegenerator 54 renders switch 52 nonconductive again and the system may berepeatedly recycled.

- It will be readily appreciated that the total potential differenceseen across the aperture 44, 44a of the antenna, for the same successiveinstants of time as described above, may be illustrated as in therespective FIGS. 5b, 6b, 7b, and 8b. It is seen that the potential atthe antenna aperture due to the real source 50 (or A) is progressivelyeaten away by the travel of the wave due to the second or effectivesource B started toward the aperture 44, 44a when switch 52 isconductive and then reflected at the aperture where radiation occursultimately to effect substantial discharge of the line formed byconductors 42 and 420, the wave having returned to be absorbed in theresistances 56, 56a.

As noted previously, it is the instant of reflection of the wave of theeffective source B at the distance L along conductors 42 and 42a (theaperture of transmitter-antenna I) that is of prime interest. Because ofthe finite characteristic impedance r of the transmitterantenna system,the leading edge of the V/2 wave launched into the aperture or mouth ofthe antenna, which is in effect an open circuit, reverses in directionof flow while maintaining its previous polarity. Radiation into space ofan impulse signal proportional to dV/dt must occur at this instant oftime. No further radiation can obtain until after switch 52 is recycledand conductors 42 and 420 are recharged. As noted above, if theresistance r of the sum of resistors 56 and 56a is made equal to thecharacteristic impedance of the transmission line system, the reflectedwave front finally terminates in resistors 56, 56a and the potentialdifference across the entire line drops to substantially zero and thenbegins to recharge to approximately rv/R volts, recharging requiring2rC, seconds.

It will be appreciated by those skilled in the art that alternative waysare available for producing cyclic storage of energy in the transmissionline systems of FIGS. 3 and 4 and for its release for propagation alongthe transmission line to an antenna radiating aperture. For example,mercury-wetted reed switches may be employed of the type disclosed inthe H. Maguire U.S. Pat. application Ser. No. 852,656 for a Coaxial LineReed Switch Fast Rise Time Signal Generator with Attenuation MeansForming an Outer Section of the Line, filed Aug. 25, 1969, issued Feb.16, 1971 as U.S. Pat. No. 3,564,277, and assigned to the Sperry RandCorporation. Switches of the type disclosed in the GP. Ross et al U.S.Pat. application Ser. No. 843,945 for a High Frequency Switch, filedJuly 23, 1969, issued Mar. 9, 1971 as U.S. Pat. No. 3,569,877, and alsoassigned to the Sperry Rand Corporation, may be employed. The transistorswitch 52 may be organized in the transmitter-antenna system in such away that it is part of a selfexciting circuit. Such arrangements andothers applicable in the present invention are discussed in theforementioned G.F. Ross et a1 U.S. Pat. application Ser. No. 46,079 fora Balanced Radiator System.

The omnidirectional bicone antenna 8 intended to receive impulsetransmissions from transmitterantenna l is seen in FIGS. 1 and 9 mountedwithin a cylindrical radome 63 on the roof 6a of the cooperating fleetvehicle. An alternative type of antenna illustrated in FIG. 10 may beused if the pulse repetition rate approaches the resonant frequency ofthe antenna. However, the omnidirectional antenna element shown in FIG.9 maximizes the response amplitude without excessively increasing theresponse time of the received signal. The antenna is composed of aconducting cone 61 with its apex pointed downwardly and supported so asto pend from the inner surface of a flat top portion 63a of dielectricradome 63. The apex of cone 611 is coupled to the inner conductor 62 ofa short coaxial cable cooperating with the concentric outer conductor62a. Conductors 62 and 62a comprise a coaxial transmission lineprojecting through a hole in the roof 6a of the fleet vehicle 6. In thisway, the roof 6a forms a ground plane for antenna 8 in the conventionalmanner, enhancing the energy collecting efficiency of antenna 8.

Filter 65 is used to eliminate undesired relatively low frequencysignals and to pass received impulse wave trains to a detector circuitfeaturing diode 69, which diode is coupled to ground and through seriesresistors 67 and 68 to a suitable source of bias voltage (not shown).Diode 69 is preferably a tunnel diode or other high speed diode adaptedto serve as an impulse detector. A suitable diode has a negativeresistance currentvoltage characteristic such that, under proper bias,the diode response to the arrival of impulse emissions from thetransmitter-antenna configuration 1 is to move abruptly into its regionof instability, causing it to become highly conductive.

In this manner, a current impulse of somewhat greater amplitude but ofconsiderably longer duration is generated by tunnel diode 69 and iscoupled to the input of one shot multivibrator circuit 70; the longerduration, higher energy signal is required for reliable triggering ofmultivibrator 70. The output pulse of multivibrator 70 is a rectangularpulse of H00 nanosecond duration, for example, which is passed to ANDgate 72. The 100 nanosecond pulse is coupled also by lead 71 to thejunction 66 between bias control resistors 67 and 68. At junction 66,the trailing edge of the 100 nanosecond pulse has the effect ofresetting diode 69 and of stopping conduction therethrough. Thus, tunneldiode 69 is reset to its original low conduction state and is preparedto receive the next arriving impulse from transmitter antennaconfiguration l which exceeds the triggering level of diode 69.Accordingly, if the transmitter-antenna configuration ll producesimpulses at an impulse repetition frequency in the vicinity of 5kilohertz, the output of multivibrator 70 is a pulse train of 100nanosecond pulses having a repetition frequency of five kilohertz.

The output of one shot multivibrator circuit 70 is coupled to AND gate72, to which a controlling signal from interrogator device 78 is alsosupplied. Interrogator device 78 may be operated regularly by a suitabledigital or other clock at intervals of five seconds, at the will of thevehicle operator, or by a command signal received, for instance, fromhead-quarters by transceiver 75, and comprises any conventional pulsegenerating device suitable for supplying an interrogation pulse ofpredetermined length for proper control of the conventional AND gate 72.The duration of the interrogation pulse may be, for example, fixed at500 milli-seconds. Thus, separated pulse trains are passed to countercircuit 73 by AND gate 72 as the vehicle moves along a serviced route.The pulses present in each such pulse train are counted by counter 73for a standard time interval, counter 73 being a conventional countercircuit of the type adapted to count incoming pulses and to transfer thecount to an encoder 74 at the end of the prescribed time interval orother condition.

A signal representing the pulse count in the predetermined time, andtherefore identifying the corresponding location of the cooperatingfleet vehicle 6, may be transmitted directly to central headquarters bythe usual voice radio communication link already present within thevehicle, for example, by transceiver 75 and omnidirectional antenna 7.

In a preferred system, the pulse count in counter 73 may beautomatically shifted out of counter 73 into a conventional encoder 74.Encoder 74 reduces the burden of transmission by transceiver 75 byconverting the pulse count into an encoded representation thereof thatis much simpler to transmit. Consequently, encoder 74 may be designed ina conventional manner also to cause transmission to the headquarterscenter of a pulse coded signal automatically identifying the particularfleet vehicle as it reports its location.

It is seen that the receiver of FIG. 9 is a wide band device; except forthe presence of high pass filter 65, which may have, for example, agigahertz cut-off frequency, the receiver would respond to any signallevel in excess of, for example, the millivolt level which might bedictated by the characteristics of a particular tunnel diode 69. Theamplitude of the received impulse at the receiving antenna 8 is, orexample, about 200 millivolts in the usual operating circumstance, avalue several orders of magnitude greater than the signals present in anurban environment due to conventional radiation sources, suchinterfering signals normally being at the microvolt level. Accordingly,although the receiver of FIG. 9 essentially accepts all signals in thepass band of filter 65, it is substantially immune to interference fromconventional radiation sources, including electrical noise signals suchas internal combustion engine ignition noise.

As has been observed in the foregoing discussion, the directivetransmitter-antenna configuration 1 shown in FIGS. 3 and 4 is capable oftransmitting a regular train of extremely short duration, high amplitudeimpulses. In a typical situation, these impulse-like signals have timedurations of 200 pico-seconds and an impulse repetition frequency in theorder of 10 kilohertz. If the voltage applied by battery 5 of FIG. 4 isassumed to be 500 volts and the source impedance 50 ohms, then the upperbound on the average power transmitted into all of space is less than Imicrowatt. The spectrum of the transmitted signal is spread over anextremely wide band width, typically 100 megahertz to gigahertz.Accordingly, the power radiated in any typical narrow communication bandis far below the thermal noise threshold of a typical receiver operatingin that band. The transmitted impulse is therefore incapable ofinterfering with the operation of standard radio communicationequipment. Indeed, the operation of the transmitter-antennaconfiguration 1 is such as not to require governmental licensing underpresent regulations.

The conical antenna used in the receiver system of FIG. 9 best optimizesthe maximum received signal and the minimum response time of any knownomnidirectional receiving antenna. Other omnidirectional antennas canalso be used when response time or amplitude limitations are not severe.In particular, when the pulse repetition frequency approaches 100magahertz, a thin film top-hat-loaded antenna such as shown in FIG. 10may be used. Such an antenna is disclosed in the G.F. Ross U.S. Pat.application Ser. No. 832,337 for a Time Limited Impulse ResponseAntenna, filed June II, 1969, issued June 22, 1971 as U.S. Pat. No.3,587,107, and assigned to the Sperry Rand Corporation. Antennas thathave a time-limited impulse response do not ring, so as to causeinterference with a succeeding input pulse.

In FIG. 10, the antenna is seen to be mounted on a ground plane 6a,which again represents the roof of the cooperating fleet vehicle, theantenna being coupled to a receiver system via coaxial line 62, 62a, asin FIG. 9. In FIG. 10, a thin resistive layer 90 formed of a thinchromium plating is positioned above and parallel to ground plane 6a.Resistive film 90 may be applied to a glass or dielectric disc 91. Thecoated plate 91 is preferably constituted of dielectric material and mayin practice be the flat portion 63a of a radome 63 fully enclosing theantenna, as illustrated in FIG. 9. In other arrangements, resistive film90 and the top disc 91 may very simply be supported upon a layer 92 ofair foamed dielectric material of conventional nature, layer 91 thusperforming the protective function of a radome.

A portion 93 of the central conductor 62 of coaxial line 62, 62a extendsthrough dielectric layer 92 and joins resistive disc 90 substantially atits center. Resistive layer 90 is preferably constructed to have aradius approximately equal to the length of the portion 93 of conductor62 found within dielectric layer 92. Typically, the resistive layer 90has a radius greater than 200 times the diameter of conductor 93.

The operation of the antenna 8 of FIG. 10 is described in the abovementioned U.S. Pat. application Ser. No. 832,337, and is understood byassuming that an impulse plane-polarized wave with its electric factororiented parallel to conductor 93 is caused to impinge on the antennafrom the left in the drawing, as indicated by arrow 94. As the incomingimpulse wave reaches the antenna, voltages are established betweenground plane 6a and the resistive layer 90. When the incoming impulsewave first reaches the conductor 93, impulsive currents are produced ateach infinitesimal segment of conductor 93 and flow downward toward theentrance to coaxial line 62, 62a. The induced currents also flow upwardtoward resistive layer 90, where any such are absorbed. The usefuldownward flowing current passes without reflection into coaxial line 62,

62a and is detected by diode 69. Because the upward flowing current isabsorbed by resistive layer 90, it cannot be reflected downwardsubsequently to appear in coaxial line 62, 62a and thus distortion whichwould ordinarily be produced is substantially eliminated.

Accordingly, the monopole receiving antenna of FIG. 10 is equipped witha monopole conductor portion 93 of a length such that the voltageinduced at its upper tip travels to its base adjacent ground plane 6a ina time substantially equal to the duration of the received impulse. Themonopole element 93 extends between the aperture ground plane 6a and thethin resistive layer 90, the latter having a surface resistivitysubstantially equal to the impedance of free space. The re sistive layeralso has a radius at least equal to the length of the monopole 93 so asto provide an essentially reflectionless termination for monopole 93 andsubstantially to eliminate distortion of the received electromagneticimpulse.

It is seen that the invention is an impulse radio communication systemusing very low total energy level, coded, transmitted impulses having aspectral contant spread over a very wide band so as to make nosignificant contribution to the background electrical noise level andthus operating well below levels interfering with government controlledradio transmissions. The transmitters of the system are adapted toexcite vehicle borne impulse recievers for identifying fleet vehicles,at the same time identifying their presence at selected locations alongroutes traversed by the vehicles. Coded identify and location data isautomatically transmitted to a central headquarters location where itmay be processed and stored for deriving instructions which may beissued to drivers of individual vehicles by conventional broadcastcommunication equipment. The impulse transmitter and impulse receiverelements are of very simple nature and are otherwise inexpensive ofinstallation maintenance, and operation, adapting readily to cooperativeuse with conventional transceiver equipment already in use in many typesof fleet vehicles. The invention has great versatility, being adaptableto use, for example, with manual monitoring and manual map posting atheadquarters, along with voice communication of instructions where thevehicle fleet to be monitored is small. On the other hand, the inventionlends itself to use at headquarters with complex data processingequipment for performing one or more of the same or other functions in amultiple unit fleet. It will be apparent to those skilled in the artthat the central processing equipment, including displays, may begenerally similar to those employed in air line or rail road trafficcontrol systems.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than of limitation and that changes within the Ellpurview of the appended claims may be made without departing from thetrue scope and spirit of the invention in its broader aspects.

We claim:

1. A vehicle monitoring system for signalling to a central station thepresence of a cooperating vehicle at any one of a plurality ofpredetermined locations comprising:

a plurality of independent electromagnetic impulse transmitter means forcontinuously illuminating said respective discrete predeterminedlocations, each said impulse transmitter means being characterized bytransmitting signal impulses having a distinctive impulse repetitionfrequency for identifying its respective predetermined location,electromagnetic impulse receiver means aboard said cooperating vehiclefor counting the number of said signal impulses received in apredetermined time interval,

encoder means responsive to said electromagnetic impulse receiver meansfor forming an encoded representation of said number of said signalimpulses received in a predetermined time, and

vehicle borne transmitter means responsive to said encoder means fortransmitting said encoded representation to said central station foridentification thereat of said location of said vehicle when illuminatedby said electromagnetic impulses- 2. Apparatus as described in claim 1including means cooperating with said vehicle borne transmitter meansfor transmitting representations to said central station foridentification thereat of said vehicle.

3. Apparatus as described in claim 1 wherein said vehicle bornetransmitter means further includes omnidirectional antenna means.

4. Apparatus as described in claim 1 wherein said electromagneticimpulse receiver means comprises:

electromagnetic impulse receiver antenna means,

biased diode means for converting said electromagnetic energy impulsescollected by said receiver antenna means directly into amplified currentimpulses, and

means for converting said amplified current impulses directly intooutput pulse signals having durations substantially longer than saidamplified current impulses.

5. A communication system adapted for receiving impulse transmissionshaving a distinctive repetition frequency characterizing a particularvehicle location comprising:

vehicle mounted omnidirectional antenna means for collecting saidimpulse transmissions when said vehicle is at said location,

biased diode circuit means connected to said omnidirectional antennameans for converting said collected impulse transmissions directly intoamplified current impulses,

pulse shaping means connected to said diode circuit means for formingcorresponding output pulse signals having durations substantiallygreater than said amplified current impulses, counter means for countingthe number of said output pulse signals occurring in a predeterminedtime for identifying said location of said vehicle, and transmittermeans responsive to said counter means for encoding the output of saidcounter means and for transmitting said encoded output of said countermeans directly to a central station for identification thereat of saidparticular vehicle location. 6. Apparatus as described in claim 5further including means cooperating with said transmitter means fortransmitting representations to said central station for identificationthereat of said vehicle.

7. Apparatus as described in claim 5 wherein said pulse shaping meansconnected to said biased diode circuit means for forming correspondingoutput pulse signals having durations substantially greater than saidamplified current impulses is coupled in feed back relation to saidbiased diode circuit means for assuring extinction of'current flowthrough said biased diode circuit means at a delayed time after thestart of each amplified current pulse.

8. Apparatus as described in claim 7 wherein said counter means forcounting the number of said output pulse signals occurring in apredetermined time comprises:

a gate circuit having input, output, and control means,

means for coupling said output pulse signals to said gate input means,

interrogator control means coupled to said control means for causingsaid gate to conduct for a predetermined time period, and

pulse counter means connected to said gate output means for countingsaid output pulse signals passed by said gate within said predeterminedtime. 9. Apparatus as described in claim 8 comprising: encoder meansresponsive to said counter means for generating an encodedrepresentation of the number of said output pulse signals passed by saidgate within said predetermined time, and

omnidirectional radiating transceiver means responsive to said encodermeans.

10. Apparatus as described in claim 5 wherein said biased diode circuitmeans comprises:

bistable semiconductor diode means,

circuit means for biasing said bistable semiconductor diode meanssubstantially at the current conduction condition thereof, and

means for coupling said impulses collected by said antenna means to saidbistable semiconductor diode means for causing impulse currentconduction thereof.

1. A vehicle monitoring system for signalling to a central station thepresence of a cooperating vehicle at any one of a plurality ofpredetermined locations comprising: a plurality of independentelectromagnetic impulse transmitter means for continuously illuminatingsaid respective discrete predetermined locations, each said impulsetransmitter means being characterized by transmitting signal impulseshaving a distinctive impulse repetition frequency for identifying itsrespective predetermined location, electromagnetic impulse receivermeans aboard said cooperating vehicle for counting the number of saidsignal impulses received in a predetermined time interval, encoder meansresponsive to said electromagnetic impulse receiver means for forming anencoded representation of said number of said signal impulses receivedin a predetermined time, and vehicle borne transmitter means responsiveto said encoder means for transmitting said encoded representation tosaid central station for identification thereat of said location of saidvehicle when illuminated by said electromagnetic impulses.
 2. Apparatusas described in claim 1 including means cooperating with said vehicleborne transmitter means for transmitting representations to said centralstation for identification thereat of said vehicle.
 3. Apparatus asdescribed in claim 1 wherein said vehicle borne transmitter meansfurther includes omnidirectional antenna means.
 4. Apparatus asdescribed in claim 1 wherein said electromagnetic impulse receiver meanscomprises: electromagnetic impulse receiver antenna means, biased diodemeans for converting said electromagnetic energy impulses collected bysaid receiver antenna means directly into amplified current impulses,and means for converting said amplified current impulses directly intooutput pulse signals having durations substantially longer than saidamplified current impulses.
 5. A communication system adapted forreceiving impulse transmissions having a distinctive repetitionfrequency characterizing a particular vehicle location comprising:vehicle mounted omnidirectional antenna means for collecting saidimpulse transmissions when said vehicle is at sAid location, biaseddiode circuit means connected to said omnidirectional antenna means forconverting said collected impulse transmissions directly into amplifiedcurrent impulses, pulse shaping means connected to said diode circuitmeans for forming corresponding output pulse signals having durationssubstantially greater than said amplified current impulses, countermeans for counting the number of said output pulse signals occurring ina predetermined time for identifying said location of said vehicle, andtransmitter means responsive to said counter means for encoding theoutput of said counter means and for transmitting said encoded output ofsaid counter means directly to a central station for identificationthereat of said particular vehicle location.
 6. Apparatus as describedin claim 5 further including means cooperating with said transmittermeans for transmitting representations to said central station foridentification thereat of said vehicle.
 7. Apparatus as described inclaim 5 wherein said pulse shaping means connected to said biased diodecircuit means for forming corresponding output pulse signals havingdurations substantially greater than said amplified current impulses iscoupled in feed back relation to said biased diode circuit means forassuring extinction of current flow through said biased diode circuitmeans at a delayed time after the start of each amplified current pulse.8. Apparatus as described in claim 7 wherein said counter means forcounting the number of said output pulse signals occurring in apredetermined time comprises: a gate circuit having input, output, andcontrol means, means for coupling said output pulse signals to said gateinput means, interrogator control means coupled to said control meansfor causing said gate to conduct for a predetermined time period, andpulse counter means connected to said gate output means for countingsaid output pulse signals passed by said gate within said predeterminedtime.
 9. Apparatus as described in claim 8 comprising: encoder meansresponsive to said counter means for generating an encodedrepresentation of the number of said output pulse signals passed by saidgate within said predetermined time, and omnidirectional radiatingtransceiver means responsive to said encoder means.
 10. Apparatus asdescribed in claim 5 wherein said biased diode circuit means comprises:bistable semiconductor diode means, circuit means for biasing saidbistable semiconductor diode means substantially at the currentconduction condition thereof, and means for coupling said impulsescollected by said antenna means to said bistable semiconductor diodemeans for causing impulse current conduction thereof.